1. Field of the Invention
This invention relates to a capacitance type dynamic quantity sensor having a movable electrode and a fixed electrode for detecting a dynamic quantity applied thereto based on a change in capacitance between the movable electrode and the fixed electrode.
2. Description of the Related Art
JP-A-11-326365 proposes a capacitance type dynamic quantity sensor. The dynamic quantity sensor has a displacement portion (spring portion) connected to a support substrate (semiconductor substrate), a movable electrode connected to the displacement portion to be moved together with the displacement portion, and a fixed electrode provided facing the movable electrode.
The displacement portion is composed of a first beam connected to and supported by the support substrate at a middle part thereof, a second beam provided separately from and in parallel with the first beam, and connection parts connecting both ends of the first and second beams, thereby forming a frame shape. The movable electrode is connected to a middle part of the second beam, and is displaceable together with the second beam.
The support substrate is formed with electrode pads that are electrically connected to the movable electrode and the fixed electrode. Generally, the electrode pads are connected to an external circuit by wire bonding.
For example, when a dynamic quantity such as acceleration is applied to the dynamic quantity sensor, the first and second beams of the displacement portion perform detecting vibration with the connection part as a fixed end that extends perpendicularly to the longitudinal direction of the beams. The movable electrode is then displaced in accordance with the detecting vibration to change a capacitance between the movable electrode and the fixed electrode. The dynamic quantity is detected based on this change in capacitance.
However, according to studies and experiments by the inventors, it is revealed that the conventional dynamic quantity sensor described above has the following problem. That is, when wire bonding is performed to the electrode pads, the wire bonding involves vibration so that the vibration is applied to the displacement portion. If the frequency of the wire bonding coincides with the natural frequency of the beams of the displacement portion, the beams of the displacement portion may resonate so that the connection part hits against a peripheral portion thereof. This can result in damage to the displacement portion.
To solve this problem, it is conceivable that the natural frequency of the first and second beams is controlled not to coincide with the frequency of the vibration of the wire bonding. In this case, however, vibration characteristics of the displacement portion for detecting the dynamic quantity would also be changed at a practical usage.
The present invention has been made in view of the above problem. An object of the present invention is to restrain a displacement portion of a capacitance type dynamic quantity sensor from being damaged by wire bonding while securing vibration characteristics of the displacement portion for detecting dynamic quantity.
According to the present invention, a displacement portion of a capacitance type dynamic quantity sensor is composed of a first beam connected to a support substrate, a second beam provided separately from and in parallel with the first beam and connected to a movable electrode, and a beam connection part connecting the first and second beams at ends thereof. The first and second beams perform detecting vibration with the beam connection part working as a fixed end in a direction perpendicular to a longitudinal direction of the displacement portion, when a dynamic quantity is applied to the displacement portion. The first and second beams further can perform flexural vibration to bend in an identical direction with the beam connection part working as a free end. A shape of the beam connection part is adjusted so that a natural frequency of the displacement portion at the flexural vibration is different from a frequency of vibration that is applied to the displacement portion by wire bonding.
The present invention is based on a finding that the displacement portion performs vibrations (detecting vibration and flexural vibration) different from each other at a practical operation for detecting the dynamic quantity and at the wire bonding. The shape adjustment of the beam connection part hardly affects the first and second beams that dominate vibration characteristics for detecting the dynamic quantity. Therefore, the displacement portion can be prevented from being damaged by the wire bonding while securing the vibration characteristics for detection.